Heat of Vaporization Calculator
Calculate heat of vaporization, transferred heat or changed mass, with kg, grams and pounds and a stated latent-heat value.
Estimate the latent energy for vaporizing a liquid or condensing a vapour, or solve for the mass changed by a known energy transfer. The calculation uses the specific latent heat at the stated conditions. It distinguishes a positive energy magnitude from the signed transfer into or out of the material.
How to use this tool
- Choose energy, changed mass or latent heat as the unknown.
- Choose absorbed energy for vaporization or released energy for condensation.
- Enter the two known quantities and select their units.
- Check that the latent heat matches the material, temperature and pressure before using the result.
Formula
|Q| = m × Lv; m = |Q| ÷ Lv; Lv = |Q| ÷ m
Lv is the specific latent heat of vaporization in kJ/kg. The default 2,256 kJ/kg is an approximate water value at its 100°C boiling point under ordinary atmospheric pressure. Water evaporating at a different temperature requires a different value; this is not an evaporation-rate model.
Example
Vaporizing 2 kg of water already at the specified boiling condition with Lv = 2,256 kJ/kg requires 4,512 kJ, equal to 1.253333 kWh. Condensing the same mass at the same condition transfers −4,512 kJ to the material, meaning that energy is released to its surroundings.
Frequently asked questions
Does this include bringing cold water to a boil?
No. Add the sensible energy for raising its temperature, any container heating and losses to obtain a broader energy estimate.
Why does pressure matter?
The phase-change temperature and enthalpy difference depend on the thermodynamic state. Use a property value corresponding to the actual pressure and temperature.
Does the result say how long evaporation takes?
No. Time depends on heat supply and mass-transfer conditions. A latent-energy calculation alone cannot predict drying time or evaporation rate.
Can I use grams or pounds?
Yes. The mass is converted internally to kilograms before applying the latent heat. The returned mass uses your selected unit, avoiding a separate manual conversion.
Sources
Last updated: 2026-10-30